1. Field of the Invention
Embodiments of the present invention relate, in general, to systems and methods for communication among components within wireless regional area networks and particularly to the selection of common control channels among overlapping wireless regional area network cells.
2. Relevant Background
In a wireless network comprising multiple overlapping cells in which the cells are working on different channels, frames must be synchronized; this is called multiple-channel synchronization (“MCS”). In addition to frame synchronization, control information must also be transmitted among multiple overlapping cells. Dynamic Frequency Hopping (“DFH”) is an example of MCS meaning it requires such frame synchronization and control information exchange. DFH incorporates non-traditional dynamic channel allocation with slow frequency hopping. One significant application of DFH is found in the operation of what is referred to in the art as a Wireless Regional Area Network (“WRAN”).
A WRAN cell consists of a Base Station (“BS”) and the associated customer Premise Equipments (“CPEs”) that communicate to the BS via a fixed point-to-multi-point radio air interface (i.e. an antenna). Overlapping WRAN cells working on the same channel coordinate operations via coexistence beacon protocol (“CBP”) messages. CBP is a best-effort protocol based on coexistence beacon transmissions to cope with self-interference issues and avoid interference with incumbent DFH community users. To operate, WRAN cells must satisfy two apparently conflicting requirements: (1) assure the Quality of Service (“QoS”) satisfaction for WRAN services and (2) continue to provide reliable and timely frequency spectrum sensing. Current sensing requirements state that incumbent signals shall be detected by WRAN devices with no more than a 2 second delay. Thus, a WRAN cell must perform sensing on a working channel at least every 2 seconds. Since a channel that is to be sensed cannot be used for data transmission, a cell operating consistently on a single channel must interrupt data every 2 seconds for sensing. Such a non-hopping mode leads to periodic interruptions and can significantly decrease system throughput and impair QoS.
Generally, the key concept behind this intelligent type of frequency hopping is to adjust or create frequency hopping patterns based on interference measurements. DFH uses slow frequency hopping and adaptively modifies the utilized frequency hopping pattern based on rapid frequency quality measurements, sometimes referred to as QoS measurements. This technique combines traditional frequency hopping with dynamic channel allocation where a channel is one frequency in a frequency hop pattern. The continuous modification of frequency hop patterns is based on measurements representing an application of dynamic channel allocation to slow frequency hopping. Modifications are based on rapid interference measurements and calculations of the quality of frequencies used in a system by all CPEs and BSs. The target of these modifications is tracking the dynamic behavior of the channel quality as well as of interference.
DFH differs from conventional frequency hopping in the way the patterns are built. Instead of using random or pre-defined repetitive hopping patterns, DFH patterns are generated for active users on the fly. In this manner, the hopping patterns can be adjusted to adapt to interference changes. In DFH communication, components of a WRAN cell (the CPEs) hop over a set of channels. During operation on a working channel, sensing is performed in parallel on the intended next working channels. After 2 seconds, a channel switch takes place: one of the intended next working channels becomes the new working channel, and the channel previously used is vacated. Hence, an interruption is no longer required for sensing. Obviously, efficient frequency usage and mutual interference-free spectrum sensing can only be achieved if multiple neighboring overlapping WRAN cells operating in the DFH mode coordinate their hopping behavior. This coordination must include frame synchronization.
A DFH community is a non-empty set of neighboring WRAN cells following a common protocol that supports a coordinated DFH operation. This coordinated operation ensures mutual interference-free channel sensing and minimizes channel usage while applying DFH phase-shifting. A DFH community has one leader and, possibly, some community members. A DFH community also requires MCS if it is to exist among other DFH communities.
As previously discussed, CBP is a best-effort type of transmission that copes with self-interference issues among overlapping WRAN cells. The basic mechanism of CBP works as follows. BSs of neighboring cells schedule a coexistence window at the end of every Media Access Control (“MAC”) frame (synchronized among BSs). During a coexistence window, neighboring BSs communicate using coexistence beacons. CBP was developed for constant channel assignment. Alternatively, in DFH community mode the channel assigned for transmission to individual cells varies with time.
FIG. 1 shows a high level view 100 of two overlapping WRAN cells 110, 120 operating on different working channels as is known in the prior art. Each WRAN cell 110, 120 includes a base station 140, 130 and operates on its own working channel. In this case the left most WRAN cell 110 operates on channel C1 and the right most WRAN cell 120 operates on channel C2. Within the overlap area 150 two CPEs 115, 125 exist. As is known in the prior art, each of these CPEs continues to operate on the working channel of the WRAN cell with which it is associated. In this example, the first CPE 115 operates on channel C1 while the second CPE 125 operates on channel C2. Within this overlap area 150 the CPEs are not synchronized thus presenting an opportunity for collisions and reduced performance. Furthermore, since the CPEs 115, 125 are operating on different working channels, they cannot communicate to one another to aid in coexistence using CBP despite their close proximity.
So while CBP works well with synchronization of overlapping WRAN cells operating on the same channel and can facilitate self coexistence among these overlapping cells, it cannot be used for multiple-channel synchronization. Frame synchronization among neighboring systems and control information exchange operating on different channels remains a challenge.